MIPS64: Fix 'Fix keyed element access wrt string wrappers'.
MIPS64: Fix 'Fix keyed element access wrt string wrappers'.

Port 01f40e6ad6c8137708955494987857fbe489616b

BUG=v8:4296
LOG=n

Review URL: https://codereview.chromium.org/1233923002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29624}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh